KisMAC is a free, open source network stumbler that operates in passive mode, allowing the program to run completely invisibly without sending any probe requests (unlike active scanners).

In addition to standard stumbler logging and graphing features, KisMAC reveals cloaked SSIDs and can perform deauthentication, packet injection, and several other attacks against WEP/WPA encrypted packets. GPS mapping is also included.

KisMAC is the only stumbler to support Apple's Airport Extreme card in passive mode. It is also compatible with original Airport cards and several third party cards (see website).